The [M-1]− ion in the negative chemical ionization mass spectra of diazepam and nordiazepam

The [M-1]− ion sometimes observed in the methane electron capture/negative chemical ionization mass spectra of diazepam and nordiazepam is [M-H]−. Generation of the [M-H]− ion is caused by trace amounts of oxygen in the ion source. For nordiazepam the Hż lost from the [M]−ż ion is that on the amide nitrogen. For diazepam the Hż lost from the [M]−ż ion is that on the carbon adjacent to the amide carbonyl. The [M-1]− ion generated by negative chemical ionization is structurally different from the [M-1]+ ion generated by positive electron ionization. At ion source temperatures greater than 150°C, the methane electron capture/negative chemical ionization mass spectra of diazepam and nordiazepam consist principally of [Cl]− ions. A comparison of the relative sensitivities of various analogs of diazepam and nordiazepam suggests that electron capture occurs at the imine bond.